Microscopic foreign matters or particles which cause a reduction in the yield during the manufacture of semiconductor devices of highly integrated circuits comprise metals, organic or inorganic materials or mixtures thereof, which are birefringent materials in a broad sense when considered optically. When a linearly polarized wave impinges upon a birefringent material, the wave is converted into a linearly polarized wave having a polarization plane which is orthogonal to that of the incident linearly polarized wave for part or all of the power of either transmitted or reflected wave, and is generally changed into an elliptically polarized wave. Obviously, the polarization of the transmitted or reflected wave changes depending on the angle formed between the polarization plane of the incident linearly polarized wave and the optical axis of the birefringent material. By irradiating such a microscoptic foreign matter with polarized light and analyzing light which comes from the foreign matter as a result of reflection, refraction or birefringence, it is possible to detect the presence of such a microscopic foreign matter. A technique which employs the irradiation of foreign matter with polarized light to detect the presence of foreign matter is already implemented in practical use, allowing microscopic foreign matter or defect which has been hardly detectable in the prior art to be detected with a high S/N ratio, as disclosed in Japanese Laid-Open Patent Application No. 6-317534, for example, which will be briefly summarized below.
Referring to FIG. 1, a laser beam radiated from a laser source 1 is passed through a polarizer 2 and a polarized beam splitter 5 to be separated into a pair of s-polarization and p-polarization which are linearly polarized in mutually orthogonal directions. Frequencies of these polarized light beams are modulated in accordance with mutually different shift frequencies .omega. and (.omega.+.DELTA..omega.) in frequency shifters 9, 11, respectively,. The modulated polarized light beams are combined together in a polarization beam splitter 12 to produce a single laser beam 12a having a pair of linear polarizations in mutually orthogonal directions which have a relative shift frequency .DELTA..omega. therebetween. The combined beam is then separated by a polarization beam splitter 15 into a p-polarization 15a and an s-polarization 15b, which are then reflected by reflecting mirrors 18, 19, respectively, so as to irradiate a common spot 20 to be inspected on the surface of a specimen 21 disposed on a movable stage 22 from two distinct directions.
In a region of the surface of the specimen 21 which is free from foreign matter, p-polarized light which is reflected by the mirror 18 to impinge on the surface of the specimen is reflected to be incident on a polarizer 27. The incident p-polarized light is cut off if the polarization axis of the polarizer 27 is chosen to be at right angles to the p-polarization. On the other hand, in the same region, s-polarized light from the mirror 19 is reflected by the surface of the specimen and cannot impinge on the polarizer 27 which is located on the side as the mirror 19 with respect to a plane passing through the spot 20 and that is perpendicular to a plane containing the points of reflections on the mirrors 18, 19 and the spot 20. Accordingly, there is no output from a photoelectric transducer 29 for the region which is free from microscopic foreign matter. However, when microscopic foreign matter or defect which is present on the surface of the specimen is irradiated at spot 20 by rays of p- and s-polarizations from the mirrors 18 and 19, there is an incidence on the polarizer 27 of 0-order diffracted light (reflected light) 23 having s-polarization which is changed from the p-polarization and scattered light 25 having the s-polarization. It will be seen that polarization components of diffracted light 23 and scattered light 25 in the direction of the polarization axis of the polarizer 27 (that is, .omega. modulation component and (.omega.+.DELTA.) modulation component) pass through the polarizer 27 to be incident on the transducer 29. In this manner, a difference frequency component between diffracted light and scattered light, which will be hereafter referred to as "beat signal", is optically heterodyned or detected by the transducer 29 while the irradiated spot 20 is scanned across the foreign matter. A beat signal processing unit 30 is connected to the output of the transducer 29 and determines the width of the beat signal, thus calculating the size of the foreign matter. The polarization is only subject to a change due to the presence of foreign matter or defect, and only that light which has its linear polarization influenced by the presence of foreign matter or defect passes through the polarizer 27 to be incident as an interference input on the transducer 29 to produce the beat signal. Accordingly, a foreign matter or defect to be detected which is located on a surface to be inspected can be detected and/or inspected with a high S/N ratio while discriminating it from any other pattern other than foreign matter or defect such as a circuit pattern or the like. However, this technique only enables the detection of the intensity of the beat signal obtained by the optical heterodyne detection, but information concerning the nature of the foreign matter cannot be obtained.
The apparatus for inspecting microscopic foreign matter as described is arranged to cause the pair of p- and s-polarizations which are linear polarizations in mutually orthogonal directions to irradiate the common spot 20 to be inspected which is located on the surface of the specimen and which has a diameter on the order of 0.3 .mu.m in two distinct directions. It is to be noted that an adjustment of the reflecting mirrors 18, 19 to bring the irradiating spots of the p- and s-polarizations into coincidence with each other on the surface of the specimen and an alignment of the optical axes of the p-and s-polarizations relative to each other are not a simple matter to implement. The described apparatus may be used in combination with a separate analyzer such as EPMA (Electron Probe Micro-Analyzer) which uses an electron beam, and in such instance, because the apparatus used for inspection of foreign matter requires that the reflecting mirrors 18, 19 be disposed on the opposite sides of the plane extended from the beam splitter 15, it is possible that this restricts or removes the freedom of disposing elements of the analyzer which is to be combined with the apparatus.
In the described apparatus, the laser beam is separated into the pair of linear polarizations in orthogonal directions or p- and s-polarizations, one of which is used as probing light to be used for detection while the other is used as reference light, and thus the powers of the both are at the proportion of 1:1. The probing light is scattered by foreign matter or defect present on the surface of the specimen while being changed in polarization, but the proportion of the scattered light is as low as several percents. Thus, the luminous energy of the detection signal is low, as is the signal intensity. To increase the luminous energy of the detection signal, it is necessary to construct an optical system which provides a greater proportion of luminous energy for the probing light.